Rotating crystals of magnetic Janus colloids.

Monodisperse magnetic colloids are found to self-assemble into unusual crystals in the presence of rotating magnetic fields. First, we confirm a predicted phase transition (S. Jäger and S. H. L. Klapp, Soft Matter, 2011, 7, 6606-6616), directly coupled to the dynamic transition of single particle motion, from a disordered state to a hexagonal crystal. Next, going beyond what had been predicted, we report how hydrodynamic coupling produces shear melting, dislocations, and periodically mobile domain boundaries. These uniform magnetic colloids, whose structures are modulated in situ using the protocols described here, demonstrate a strategy of stimulus-response in the colloid domain with potential applications.